Issue #11303: Added shortcuts for utf8 and latin1 encodings.

Documented the list of optimized encodings as CPython implementation
detail.

Doc/library/codecs.rst

@@ -904,6 +904,15 @@ is meant to be exhaustive. Notice that spelling alternatives that only differ in
 case or use a hyphen instead of an underscore are also valid aliases; therefore,
 e.g. ``'utf-8'`` is a valid alias for the ``'utf_8'`` codec.
 
+.. impl-detail::
+
+   Some common encodings can bypass the codecs lookup machinery to
+   improve performance.  These optimization opportunities are only
+   recognized by CPython for a limited set of aliases: utf-8, utf8,
+   latin-1, latin1, iso-8859-1, mbcs (Windows only), ascii, utf-16,
+   and utf-32.  Using alternative spellings for these encodings may
+   result in slower execution.
+
 Many of the character sets support the same languages. They vary in individual
 characters (e.g. whether the EURO SIGN is supported or not), and in the
 assignment of characters to code positions. For the European languages in

Objects/unicodeobject.c

@@ -1462,13 +1462,15 @@ PyObject *PyUnicode_Decode(const char *s,
     char lower[11];  /* Enough for any encoding shortcut */
 
     if (encoding == NULL)
-        encoding = PyUnicode_GetDefaultEncoding();
+        return PyUnicode_DecodeUTF8(s, size, errors);
 
     /* Shortcuts for common default encodings */
     if (normalize_encoding(encoding, lower, sizeof(lower))) {
-        if (strcmp(lower, "utf-8") == 0)
+        if ((strcmp(lower, "utf-8") == 0) ||
+            (strcmp(lower, "utf8") == 0))
             return PyUnicode_DecodeUTF8(s, size, errors);
         else if ((strcmp(lower, "latin-1") == 0) ||
+                 (strcmp(lower, "latin1") == 0) ||
                  (strcmp(lower, "iso-8859-1") == 0))
             return PyUnicode_DecodeLatin1(s, size, errors);
 #if defined(MS_WINDOWS) && defined(HAVE_USABLE_WCHAR_T)
@@ -1670,15 +1672,19 @@ PyObject *PyUnicode_AsEncodedString(PyObject *unicode,
     }
 
     if (encoding == NULL)
-        encoding = PyUnicode_GetDefaultEncoding();
+        return PyUnicode_EncodeUTF8(PyUnicode_AS_UNICODE(unicode),
+                                    PyUnicode_GET_SIZE(unicode),
+                                    errors);
 
     /* Shortcuts for common default encodings */
     if (normalize_encoding(encoding, lower, sizeof(lower))) {
-        if (strcmp(lower, "utf-8") == 0)
+        if ((strcmp(lower, "utf-8") == 0) ||
+            (strcmp(lower, "utf8") == 0))
             return PyUnicode_EncodeUTF8(PyUnicode_AS_UNICODE(unicode),
                                         PyUnicode_GET_SIZE(unicode),
                                         errors);
         else if ((strcmp(lower, "latin-1") == 0) ||
+                 (strcmp(lower, "latin1") == 0) ||
                  (strcmp(lower, "iso-8859-1") == 0))
             return PyUnicode_EncodeLatin1(PyUnicode_AS_UNICODE(unicode),
                                           PyUnicode_GET_SIZE(unicode),